Monitoring of eye motion can provide a variety of information. Sleep researchers, for example, use eye motion as an indicator of various sleep stages. Also, persons with limited muscle control can use eye motion to interact with others or to control specialized equipment. Military applications that follow eye motion for targeting purposes or vehicle control have also been developed. Eye tracking devices are even used in the video game entertainment industry, where interactive environments adjust to follow the motion of a player's eyes.
Many eye tracking devices monitor muscle activity to assess eye motion. For example, U.S. Pat. No. 5,517,021 discloses an eye tracking apparatus that detects bioelectromagnetic signals generated by the muscles that move an individual's eye. The signals are analyzed and corresponding control signals are produced as output. U.S. Pat. No. 5,422,689 discloses an eye tracking device that uses sensors to monitor electro-oculogram signals produced by eye motion. The sensors are coupled with a microprocessor that analyzes this signals to determine an operator's horizontal or vertical eye movement.
Other eye tracking devices rely on changes in light patterns to track eye motion. For example, U.S. Pat. No. 5,270,748 discloses an eye tracker that uses detection devices for determining the point of regard of an operator. Included conversion circuitry determines the position of fovea-reflected light, allowing computation of an individual's visual axis and the associated point of regard. U.S. Pat. No. 5,345,281 discloses a system that uses reflected infrared light to track the gaze of an operator's eye. The U.S. Pat. No. 5,345,281 system directs infrared light towards the eye and considers differences in infrared reflectivities between the pupil, iris, and sclera to compute eye position. U.S. Pat. No. 5,583,335 discloses an eye tracking system that includes an active matrix display. Pixels in the display are aligned with corresponding photodetectors. Axial light rays from the display pixels are reflected by the eye and detected by respective photodetectors. In turn, the array of photodetectors generates an eye-position-indicating electrical signal.
Although known detectors provide certain information about eye motion, they have limitations. In many cases, simple eye motion monitoring does not provide a complete picture. For example, eye tracking devices that monitor eye-moving muscles typically do not sense pupil action. Feedback regarding pupil contraction and dilation provides important cues during diagnostic medical procedures. Devices that do not track this pupil activity do not provide enough information for many types of medical tests. Other trackers, such as those that monitor reflected light, may provide some information about pupil action, but do not provide real-time visual images of the eye, itself. Without this visual image to provide context, electrical eye-position information may be hard to interpret and almost impossible to cross reference. Prior U.S. Pat. No. 5,892,566 was issued to the present inventor on Apr. 6, 1999, and the contents thereof are hereby incorporated by reference. While curing many of the prior art deficiencies, the '566 patent nevertheless teaches video tracking of the eye as embedded in a visual presentation system which relies on either ambient light or light from the visual system to illuminate the eye. The problem with this system is that signals from the visual presentation system may be temporarily intermittent or be of such a limited bandwidth as to make illumination of the eye unreliable for the purpose of forming an image of the eye or its structures. Similar problems stem from reliance on ambient light for illumination. Thus, it is desirable to use a dedicated light source to illuminate the eye. The present inventor has discovered that best results are obtainable when the source of illumination has a wavelength that is different than that used for the visual presentation /stimulus, thereby rendering the illumination independent of the visual signal. Infra-red or an equivalent non-visible portion of the light spectrum has been found to be a preferred source of dedicated illumination.
Additionally, the physical and operational nature of known eye-tracking devices makes them unsuitable for use in many testing environments. For example, magnetic resonance imaging ("MRI") diagnosis equipment creates an environment which is makes it impossible to use known eye-tracking devices therein.
In operation, a typical MRI apparatus relies upon hydrogen protons which have a dipole movement and therefore behave as would a magnetic compass. In MRI scanning, the MRI apparatus operates as a large magnet wherein the protons align with the strong magnetic field but are easily disturbed by a brief radio frequency pulse of very low energy so as to alter their alignment. As the protons return to their orientation with the magnetic field, they release energy of a radio frequency that is strongly influenced by the biochemical environment. The released energy as detected and mathematically analyzed for display as a two dimensional proton density image according to the signal intensity of each issue.
The magnetic coils of the MRI apparatus are permanently fixed within a large structure so as to form a large magnet with a very confining entrance known as the bore. A patient is placed upon a scanner table that is integrated with the MRI apparatus and slid into the middle of the bore.
Eye tracking equipment used during MRI scanning must not interfere with the motion of an individual within the bore. Since the bore is a low-clearance area, eye tracking equipment used therein must be streamlined: bulky items simply will not fit. Preferably, the equipment is lightweight and worn by the patient to move with the patient within the MRI apparatus.
Additionally, eye tracking devices used during MRI scanning must transmit signals in a format that is not affected by the characteristic output of the MRI apparatus. Radio frequencies used by the MRI apparatus typically disrupt signal modulation. Known eye tracking devices are not suited for use in this environment: their signals will not be transmitted clearly.
Additionally, the inner area of the bore produces a magnetic field which will draw metal items when magnetized. Known eye-tracking devices include parts that are easily magnetized and are, as a result, not suitable for use with MRI equipment.
The Applicant was further issued U.S. Pat. No. 5,414,459 entitled Fiber Optic Video Glasses and Projection System which addressed the need for eye stimulation within an MRI apparatus. The '459 device being formed from a shape and material of construction that are suitable for use within an MRI environment without the need for additional shielding.
Thus, what is needed is an eye tracking device that includes advantages of the known devices, while addressing the shortcomings they exhibit. Accordingly, the eye tracking device should be impervious to magnetic environments and the output of MRI equipment. The device should not only indicate eye motion, but should also monitor pupil state. The device should be compact enough to monitor a patient located within the bore of MRI equipment and provide diagnostic feedback that allows comparison of eye movement and brain activity. Additionally, the device should be compatible with patient relaxation equipment used during an MRI session. Lastly, the device should include a dedicated light source to illuminate the eye which has a wavelength that is different than that used for the visual presentation/stimulus, such as infrared illumination or the like, thereby rendering the illumination independent of the visual signal.